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Reading from the "auto555/G4500DX" Manual, you need to:
1. Slide the Film Selector to desired ISO - (that is the same as your camera ISO used - so, make sure you turn Off the Auto ISO feature if your camera has one).
2. Slide the Mode Selector to "A" (Auto position). This will be Auto Thyristor mode on the flash.
3. Slide the Auto F/stop Selecct Know to desired lens openning - ( so use your camera in Aperture priority mode, more in Manual to prevent changes in the camera aperture value during shooting)
Shoot, and be happy with well exposed pictures by the Auto Thyristor features of the Sunpak 555 flash.
The Sunpak 555 works great with Nikon D50, D70 cameras, as you can synchrinize flash practically at all shutter speeds. Excellent for sunny days to get narrow DOF, yet get any ambient light level contribution as desired.
For other cameras, make sure (watch out for) that Aperture Priority mode does not use shutter faster than the camera max X-sync speed - so Manual camera mode could be a better choice for a novice photographer.
In the manual there is also a list of accessories for various brand cameras, How to use Power Ratio in Manual mode, some advice on using wide lenses, how to care of your 555, and Specification section.
Minimal flash duration is 1/450 at full power, so you can safely use Auto Aperture up to 1/500 sec. Flash duration decreases with lower power setting, or lower auto mode need for poser, and the shortest flash duration is 1/17000th sec.

If the Forced flash setting is already enabled and the flash is still not working, then the issue may be due to another camera setting. Even if the Forced flash setting is enabled, the flash will still not work in the following circumstances:

When shooting MPEG movies

When shooting in the Twilight or Handheld Twilight mode

If shooting with the Burst or Multi Burst mode

Shooting with Sweep Panorama

Shooting with the Fireworks or High Sensitivity mode.

If an external flash is connected.

Please make sure that the Camera is not set to any of these modes. You can check the manual to get the information to change the setting..
If the issue persists, please reset the Camera and make sure that battery is fully charged.
If the issue persists after performing all the above mentioned troubleshooting steps, service is required.
If you feel that the above Troubleshooting steps, helped you. please rate my answer positively.
Thanks for understanding.

Nikon hot shoe is rated up to 250V (canon's is rated up to 6V) so you can use almost any tipe of flash, except maybe for old studio flashes.Vivitar 283 (older versions) is ~200V rated, and much lower on newer versions. It should work.